physically-based groundwater vulnerability assessment using physically-based groundwater...
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Risk in Water Resources Management (Proceedings of Symposium H03 held during IUGG2011 in Melbourne, Australia, July 2011) (IAHS Publ. 347, 2011).
Copyright © 2011 IAHS Press
Physically-based groundwater vulnerability assessment using sensitivity analysis methods JEAN BEAUJEAN1,2, JEAN-MICHEL LEMIEUX3, PASCAL GODERNIAUX1 & SERGE BROUYÈRE1
1 Hydrogeology & Environmental Geology, Geo3 Group, ArGEnCo Dept., Aquapôle, University of Liège, Liège, Belgium email@example.com
2 Now at: Applied Geophysics, Geo3 Group, ArGEnCo Dept., University of Liège, Liège, Belgium 3 Département de Géologie et de Génie Géologique, Université Laval, Québec, Canada
Abstract Management of water resource systems requires adequate decision making to protect the water- related functions of fundamental importance to human life, ecosystem preservation and economic development. Groundwater vulnerability assessment studies are useful tools for land-use planning and groundwater protection. A generalized physically-based method using numerical models of groundwater flow is proposed for quantifying the impact on groundwater resources to external pressures, in terms of both quantity and quality. The proposed method is based on the definition of groundwater state sensitivity and groundwater vulnerability coefficients. The vulnerability coefficient is defined as a ratio that reflects the “distance” between the current state of degradation of the water resource system and the “damaged state”. Different numerical methods are proposed to compute the sensitivity coefficients. The uses of these concepts in risk assessment for groundwater resources are discussed and the computation algorithms are illustrated using a simple, yet insightful case study. Key words vulnerability; sensitivity; physically-based; artificial recharge; risk assessment INTRODUCTION
Vulnerability assessment studies are becoming increasingly popular as they are useful tools for land-use planning and related decisions about groundwater protection. The most popular are index- based overlay methods such as DRASTIC, EPIK and GOD used to produce a gridded vulnerability map. The major advantage is their simplicity and few data requirements (Neukum et al., 2008). However, there are many similar techniques proposed in the literature creating confusion as their respective results, on a same case study, can be very dissimilar. As a need to verify the results, physically-based methods are now proposed (e.g. Frind et al., 2006). While most groundwater vulnerability assessment methods and studies have focused on contamination issues, it is now recognized that there are several other pressures that are likely to threaten the groundwater system such as predicted changes in precipitations and groundwater recharge.
We present a general and physically-based method for evaluating and quantifying the potential impact on groundwater resources of external pressures, both in terms of quantity and quality, using numerical models of groundwater flow. The proposed method is based on the definition of groundwater state sensitivity coefficients and vulnerability coefficients. The concept of sensitivity relies on the definition of physically-based indicators of changes for the groundwater state as affected by pressures. It is extended to vulnerability by introducing a new approach borrowed from socio-economical sciences that involve the concept of likelihood of falling below a threshold.
Sensitivity analysis methods are traditionally used for automatic calibration or inverse modelling, to assess the sensitivity of a model to its parameters and boundary conditions (Sykes et al., 1985), for uncertainty analysis (Jyrkama & Sykes, 2006) and for optimization. Here, the sensitivity analysis is used as a tool for decision making and aquifer management support; we assume that a numerical model is already calibrated and we use sensitivity analysis methods to evaluate the vulnerability of the state of the system to local variations in external pressures. The emphasis is put on the choice of a sensitivity analysis method with respect to the management objectives in order to generate insightful vulnerability coefficients while minimizing the computational burden. Different numerical methods are proposed to compute the sensitivity coefficients.
Physically-based groundwater vulnerability assessment using sensitivity analysis methods
Development of human activities poses a threat to functions of fundamental importance to human life, ecosystem and economic development, assured by groundwater systems. They represent first elements of causal links which lead to quantity and quality stresses on groundwater reservoirs. Prior to the development of a groundwater vulnerability assessment method (GWVA), it is necessary to identify which are the stress factors that will be considered and which are the affected characteristics of the groundwater resource. A general framework adopted by the European Community is the Drivers-Pressures-State-Impacts-Responses (DPSIR) approach (Kristensen, 2004) that describes the interactions between society and the environment. It is used to analyse the relationships between stress factors, groundwater characteristics and resulting possible impacts. It generalizes the concept of groundwater vulnerability in a greater diversity of settings by including any kind of stress factors that can affect groundwater. This is further refined by splitting up the groundwater system (i.e. state elements) to define physical components of the state related to the pressures and the impacts to be used in a mathematical equation. This allows us to develop a conceptual model that relates the DPSIR approach with a systematic and physical representation of the groundwater system. The State component of the DPSIR chain is identified explicitly by: (1) the physical factors that relates the state variables to the pressures, called “state upstream factors” (e.g. groundwater recharge, well abstraction), and (2) the physical factors that relate the state variable with the impacts, called “state downstream factors” (e.g. reduction in baseflow, change in groundwater levels) (Fig. 1).
Fig. 1 Schematic presentation of the generalized concepts of GWVA in the DPSIR framework.
Refinement within the State component of the DPSIR chain allows us to make a further distinction between groundwater resource vulnerability (GRV) and groundwater source vulnerability (GSV). GRV reflects the vulnerability of the whole aquifer with respect to a given pressure while GSV is the vulnerability of specific components of the groundwater system with respect to given pressures (generally with a specific location in space), such as a pumping well or discharge gallery (Fig. 1).
Starting from the observation that there are others threats to the groundwater resource than local or diffuse contamination sources, an extension of the physically-based groundwater vulnerability assessment approach presented in Brouyère et al. (2001) and Popescu et al. (2004) is proposed to any kind of stress factors: The concept of groundwater vulnerability reflects the natural mechanisms and processes that make the aquifer more or less sensitive to any kind of pressure. Using the P-S-I causal chain, the concept of groundwater vulnerability is generalized to any kind of stress factor by reflecting, the easiness that the groundwater system (the “state”) leads to pressures into impacts, or, using the terminology defined earlier: the easiness that changes in the “upstream factors” transmit changes in the “downstream factors”, whatever the kind of pressure and resulting impact, thus based on the groundwater system properties only.
Jean Beaujean et al.
A general methodology for groundwater vulnerability assessment
The most general methodology consists in evaluating the sensitivity ijS of impact (I) “i” (e.g. decrease of water availability) to pressures (P) “j” (e.g. climate change) within equation (1):
i ij P
ISS ∂ ∂
This very general and specific relationship can be made clear to the case of GSV and GRV. They allow evaluating how a change in a given upstream factor (UF) “j” (e.g. changes in groundwater recharge over the basin) has knock-on effects on downstream factors (DF) “i” (e.g. baseflow to rivers) or groundwater state component (ST) “i” (e.g. safe yield), respectively:
iGS ij UF
DFSS ∂ ∂
=≡ or j
iGR ij UF
STS ∂ ∂
where GSijS is the groundwater source sensitivity and GR ijS the groundwater resource sensitivity.
The larger Sij, the more sensitive is the groundwater state, thus potentially vulnerable in the sense that it will transmit more easily a pressure (upstream factor UFj) to an impact (~DFi).
Fig. 2 Flowchart presenting the main steps of the general methodology for GWVA.
The generalized definition of groundwater vulnerability emphasizes the sensitivity of the groundwater resource/source to pressures. However, it does not take into account the margin available between the present state of the groundwater resource/source and the critical state from where it should be damaged. Luers et al. (2003) extended the concept of sensitivity (S) as defined above to a definition of system vulnerability (V) by integrating a ratio (Dw) that reflects the “distance” between the current degradation of the water system and its “damaged state”: